Recently, due to a growing concern about environmental problems, an installation of a large lithium-ion secondary battery having high energy density and excellent output performance in an electrical vehicle has been examined. A capacity per volume is important to small-size portable instruments such as cellular phones and laptop computers, and thus a graphitic material having high density is mainly used as a negative electrode active material. However, a lithium-ion secondary battery for a vehicle is large and expensive, and thus, cannot be easily exchanged in the middle of the life of the vehicle. Therefore, it is required to exhibit durability comparable to the durability of the vehicle. That is, it is expected to achieve life duration of 10 or more years (high durability). The graphitic material or a carbonaceous material having developed graphite structures is easily destroyed by expansion and constriction of crystallites due to a repetition of doping and dedoping the lithium, and thus a repetition performance of charge-discharge is poor. Therefore, they are unsuitable for negative electrode material for a lithium-ion secondary battery for a vehicle that is required to exhibit high cycle durability. On the other hand, non-graphitizable carbon material causes little expansion and constriction at the time of doping and dedoping of lithium to exhibit a high cycle durability so that it is suitable for use in cars (Patent literature 1).
Further, a charge-discharge curve of non-graphitizable carbon material is gentle compared to that of graphitic material, and the potential difference to the regulation limit of the charge is wide. Thus, a battery using non-graphitizable carbon material as a negative electrode active material can be rapidly charged compared to that using graphitic material. Furthermore, as the non-graphitizable carbon material has low crystallinity compared to graphitic material and a number of sites capable of contributing to charge and discharge, it has an excellent rapid charge-discharge performance. In connection with this, a charging time was 1 to 2 hours in small-size, portable instruments. However, a charging time of batteries for hybrid vehicles is several tens of seconds in view of energy regeneration at the time of braking, and a discharge time thereof is also several tens of seconds in view of the pressing time of the gas pedal. Therefore, particularly-excellent, rapid charge-discharge performance (input/output performance) is required in the batteries for hybrid vehicles. The negative electrode material disclosed in Patent literature 1 has high durability. However, the performance thereof is insufficient as the negative electrode material for a lithium-ion secondary battery for a vehicle in which the particularly-excellent, rapid charge-discharge performance is required, and thus, further performance improvement is expected.
Hitherto, the petroleum pitch and coal pitch are used as a carbon source of non-graphitizable carbon. The inventors of the present invention found that a carbonaceous material for a negative electrode obtained by using plant-derived char (plant-derived organic substance) as a carbon source can be doped with a large amount of active material, and thus it has promise as a negative electrode material (Patent literatures 2 and 3). However, when the plant-derived char (plant-derived organic substance) is used as a carbon source of carbonaceous material for a negative electrode, potassium found in organic, raw material had an unsuitable effect on the doping and dedoping performance. In order to solve the above problem, Patent literature 3 discloses a method for reducing an amount of potassium wherein the plant-derived char (plant-derived organic substance) is washed by acid so as to demineralize (hereinafter referred to as a demineralization in liquid-phase) (Patent literature 3). That is to say, in the method for preparing a carbonaceous material for a negative electrode wherein the plant-derived char (plant-derived organic substance) is used as a carbon source, the demineralizing treatment is required.